Nut casting method

ABSTRACT

A method for casting polymer threads (32) in a nut cavity (14) complementary to the threads (44) of a casting screw (42) to minimize the formation of air pockets and air bubbles in the polymer threads. The cavity (14) of the nut (12) has a diameter larger than the diameter of the casting screw (42). The threads (32) of the nut (12) are formed in a polymer (30) which is dispensed into the nut cavity (14) and which cures around the threads (44) of the casting screw (42) after the screw is inserted into the cavity of the nut. The method for casting the threads (32) includes the steps of closing the bottom of the nut cavity (14), thereafter filling the nut cavity with polymer (30), and thereafter inserting the casting screw (42) slowly down into the polymer.

TECHNICAL FIELD

The subject invention relates to long span lead screw and nut driveshuttle systems, and particularly to a method and apparatus for formingthreads in a polymer insert in the nut which are complementary to thethreads on the lead screw.

BACKGROUND ART

A shuttle device utilizing a long span lead screw and nut drive has beenused to transmit heavy torque and thrust loads at relatively highspeeds. One such device is shown in U.S. Pat. No. 3,670,583 in the nameof John C. Lemming, which includes a non-rotatable coactive nutoperatively mounted on a rotary screw and adapted to travel freelylinearly along the screw. The problem with conventional lead screw andnut drive assemblies is that the nut threads have to be machined tomatch the threads of the lead screw. At high speeds, the nut drive andlead screw may vibrate due to the tolerance between the lead screw andnut drive threads. Also, misalignment of the nut drive and lead screwmay occur due to the machine intolerances of the threads of the nutdrive. Further, large amounts of power are required to drive variousloads due to the short lead in a single start.

U.S. Pat. No. 4,790,971 to Brown et al teaches a method of casting nutthreads, and a corresponding apparatus, to minimize such problems. Themethod involves forming polymer threads in the cavity of a nutcomplementary to the threads on a section of the lead screw. The threadsare formed by centering a section of the screw in a nut having a cavitywith a diameter larger than the diameter of the screw threads, byfilling the cavity with a liquid polymer and by allowing the polymer todry and cure around the threads of the lead screw.

Unfortunately, when the polymer is introduced into the nut cavity aroundthe threads of the lead screw section, air pockets and air bubbles formas a part of the threaded polymer insert. The presence of air pocketsand air bubbles in the polymer insert affects not only the life of thethreaded polymer insert but also the mechanical strength of the polymerinsert.

SUMMARY OF THE INVENTION AND ADVANTAGES

An improved method of forming threads on the interior cavity extendingthrough a nut for engaging a multi-start high helix lead screw of a nutdrive and lead screw assembly comprises the following steps: closing thebottom of the cavity of the nut to render the bottom fluid-tight;thereafter filling the cavity of the nut with a viscous polymermaterial; thereafter inserting a casting screw having the threadsthereabout into the cavity to displace the polymer material to a higherlevel in the cavity; and curing the polymer material and forming thehigh helix multiple threads of the nut complementary to the threads ofthe casting screw for providing polymer nut threads having no airbubbles or open pockets.

Corresponding with this new method is a nut casting apparatus forforming threads on the interior cavity extending through a nut forengaging a multistart high helix lead screw of a nut drive and leadscrew assembly comprising a nut having an interior cavity extendingthrough the nut. The assembly is characterized by cavity closing meansfor temporarily extending across the bottom end of the cavity andrendering the bottom end fluid tight.

Accordingly, the subject method and apparatus for casting the polymernut threads allow suitable threads to be cast while reducing the numberof air bubble and air pocket imperfections in the polymer nut threadcasting.

FIGURE IN THE DRAWINGS

Other advantages of the present invention will be readily appreciated asthe same becomes better understood by reference to the followingdetailed description when considered in conjunction with theaccompanying drawings wherein:

FIG. 1 is a top view of the nut casting assembly showing the upper screwsupport member and the screw;

FIG. 2 is a cross-sectional view taken along line 2--2 of FIG. 1 showingthe nut and the upper and lower screw support members;

FIG. 3 is a cross-sectional view similar to FIG. 2, showing the polymerthreads and the casting screw;

FIG. 4 is a side view partially cut away showing the nut, the screw, theupper and lower screw support members and retaining means; and

FIG. 5 is a top view showing the upper support member, the screw and theretaining means.

EMBODIMENT DESCRIPTION OF THE PREFERRED EMBODIMENT

A nut casting apparatus for facilitating the nut casting method isgenerally shown at 10 in the figures. The apparatus 10 includes a nutgenerally indicated at 12, a cavity closing means generally indicated at54, a polymer 30, a polymer retaining means 20, a casting screwgenerally indicated at 42 and screw support means generally indicated at52, 54.

The nut casting apparatus 10 includes a nut generally indicated at 12having an interior cavity 14 extending through the nut. The nut 12usually has a cylindrical shape, though the shape of the nut has alsobeen rectangular. Indeed the nut may assume any one of a number ofshapes since the shape of the nut is not critical. The cavity 14 alsohas a cylindrical shape and is usually positioned concentrically withinthe nut 12.

The nut 12 is formed to include an upper counterbore 18 located at thetop of the nut cavity 14 and a lower counterbore 19 located at thebottom of the nut cavity. The counterbores 18,19 are cylindrical holesmachined concentric with the outer diameter of the nut 12. The bores18,19 have diameters larger than the diameter of the cavity 14. Thedepth of the counterbores 18, 19 is generally shallow, as shown in FIG.3.

The assembly 10 further includes cavity closing means generallyindicated at 54 for temporarily extending across the bottom end of thecavity 14 and rendering the bottom end fluid-tight. The cavity closingmeans 54 can involve any structure which closes the nut cavity 14 insuch a way as to allow the polymer 30 to be retained in the cavitybefore and after the casting screw 42 is inserted, and while the polymeris curing. The cavity closing means 54 is specifically defined as thelower screw support member 54, which is adapted to fit with closetolerance into the lower counter bore 19 of the nut 12. The lowersupport member 54 has a cylindrical portion 56 which engagestelescopically in the counter bore 19.

The polymer 30 may be any polymer having the necessary strength,lubricity, wearability, and flexibility for forming threads for use in anut drive and lead screw assembly.

The cavity 14 of the nut 12 includes polymer retaining means 20 forretaining the polymer insert 30 in the cavity 14 of the nut 12 after thepolymer has cured. The polymer retaining means 20 is intended to resistforces which may move the polymer insert 30 through or around the nutcavity 14. The polymer retaining means 20 functions by eliminating thesmooth surface of the nut cavity 14. The polymer retaining means 20 isdefined as helical right and left hand threads formed in the surface ofthe nut cavity 14.

The nut casting apparatus 10 further includes a casting screw generallyindicated at 42 for forming the threads 32 of the nut 12 complementaryto the threads 44 of the casting screw. The casting screw 42 may be anyscrew having a dimension and thread pattern similar to those of the leadscrew (not shown). Preferably, the casting screw 42 is taken from alength of a lead screw from a production run of lead screws to be usedwith nuts in the nut drive and lead screw shuttle assemblies. Thecasting screw 42 used for casting the nut threads 32 is ideally an endlength of the actual multi-start high-helix lead screw (not shown) to beused in the nut drive and lead screw shuttle assembly for which the nut12 is cast. For example, if a ten foot lead screw is needed for ashuttle assembly, a twelve foot screw will be procured. Two feet of thelead screw will be removed to form the casting screw 42 used for castingthe nut threads 32. Taking a length of the lead screw for the castingscrew 42 ensures that the threads 32 formed in the polymer 30 willclosely match the threads of the lead screw.

The casting screw 42 includes a convex tip 46 for resisting thecollection of air bubbles under the tip of the screw as the screw islowered into the polymer 30. The tip 46 must be formed on the castingscrew 42 prior to using the screw 42 in the casting process.

The casting screw 42 further includes polymer shrinkage compensatingmeans (not shown) for reducing the shrinking effect on the formation ofthe polymer threads 32. Since all polymers shrink somewhat while curing,it is necessary either to reform the threads after curing or tocompensate for the shrinkage. At least one of these measures isnecessary to provide suitable clearance for the lead screw. Reformingthe threads 32 after curing is difficult and expensive; so compensatingfor polymer shrinkage is preferred. The polymer shrinkage compensatingmeans is generally nickel plating on the threads 44 of the casting screw42. The plating provides for extra clearance of the unplated lead screwafter the polymer 30 has shrunk. Electroless nickel plating is used tocoat the casting screw 42 because nickel provides an even coating on thescrew. Metals such as chrome provide a less uniform coating. The platingis formed onto the casting screw 42 to a desired thickness. Thethickness of the plating depends upon the amount of shrinking that isexpected from the polymer 30. However, the maximum plating thicknessachievable with current methods is 5/1000 of an inch.

The casting screw 42 is formed to include opposing flats 48 located onthe top end of the casting screw for allowing a tool to be applied toremove the casting screw from the polymer 30 when the polymer is cured.

The nut casting apparatus 10 as set forth further includes screw supportmeans generally indicated at 52, 54 for supporting the casting screw 42in the polymer 30. The screw support means 52,54 includes a lowersupport member 54 and an upper support member 52.

The lower support member generally indicated at 54 includes lower screwcentering means 58, a concave portion 57 and a lower cylindrical portion56. The lower screw centering means 58 centers the casting screw 42 inthe nut cavity 14. The lower centering means 58 is defined as a lowerlocating counterbore 58 machined in the lower support member 54 forreceiving and engaging the outer diameter of the screw 42 as the screwpasses through the counterbore 58. The lower locating counterbore 58 iscentered in the lower cylindrical portion 56 of the lower support member54. The lower cylindrical portion 56 is integral with the lower supportmember 54. The concave portion 57 of the lower centering means 58provides clearance for the convex tip 46 of the screw 42. The concaveportion 57 is centered in the cylindrical portion 56 of the lowersupport member 54 just below the lower locating bore 58.

The upper screw support member generally indicated at 52 includes upperscrew centering means 62 and an upper cylindrical portion 68. The uppercentering means is defined as an upper locating bore 62 for receivingand engaging the outer diameter of the casting screw 42 as the castingscrew passes through the bore 62. The upper support member 52 includesan upper cylindrical portion 68 integral with the upper support member52 and adapted to engage telescopingly with close tolerance in the uppercounter bore 18 of the nut 12. The upper locating bore 62 is centeredwith respect to the upper cylindrical portion 68, and extends completelythrough the upper support member 52. When the screw 42 passes throughboth the upper locating bore 62 and the lower locating counterbore 58 itwill be centered with respect to the nut cavity 14. The upper screwsupport member 52 also includes an excess polymer cavity 64 forreceiving excess polymer 30 from the cavity 14. The excess polymercavity 64 is cylindrical in shape and extends from the top of the nutcavity 14 to the upper locating bore 62. The excess polymer cavity 64may be centered with respect to the upper cylindrical portion 68 and theupper locating bore 62. The upper support member 52 further includesopposing scallops 66 recessed in the upper locating bore 62 forreceiving further excess polymer escaping from the nut cavity 14.

In the case where the nut 12 to be cast is large, the amount of polymer30 displaced by the screw 42 may force the upper and lower supportmembers 52, 54 from engagement in the upper and lower counterbores 18,19, respectively. To prevent this problem, retaining means 70 may beprovided to retain the support members 52, 54 in engagement with the nut12. The retaining means 70 secures the upper and lower support member52, 54 to each other so that the upper and lower cylindrical portions56, 68 are retained in telescoping engagement in said upper and lowercounter bores 18, 19, respectively. Preferably, the upper and lowersupport members 52, 54 are secured to each other with the tie rods 80and the nuts 90. The upper support member 52 includes at least two holes84 for allowing the tie rods 80 to pass through. The lower supportmember 54 includes at least two threaded bores 86. The threads 82typically run along the entire length of the tie rod 80. The threads 82must at least run along the top and bottom ends of the tie rod 80.

The method for forming threads on the interior cavity 14 extendingthrough the nut 12 for engaging a multi-start high helix lead screw of anut drive and lead screw assembly generally comprises the steps of:closing the bottom of the cavity 14 of the nut 12 to render the bottomfluid-tight; thereafter filling the cavity 14 with a viscous polymermaterial 30; thereafter inserting a casting screw 42 having the threadsthereabout into the cavity 14 to displace the polymer material 30;curing the polymer material 30 and forming the threads 32 of the nut 12complementary to the threads 44 of the casting screw 42 for providingpolymer nut threads 32 having no air bubbles or open pockets.

The method first involves closing the bottom end of the cavity 14 of thenut 12. The lower screw support member 54 is coated with release agentto allow for easy removal from contact with the nut 12 at the end of theprocess. The release agent may be any suitable release agent whichprevents an object from adhering to the polymer 30 after the polymer hascured. The lower screw support member 54 is then fitted into the lowercounter bore 19 of the nut 12, thereby closing the bottom of the cavity14 of the nut.

The upper support member 52 may next be coated with release agent andfitted into the upper counter bore 18 of the nut 12. This step may beperformed as late as just after inserting the casting screw 42 into thepolymer 30.

Next, the polymer 30 should be mixed in preparation for the dispensingof the polymer into the nut cavity 14. This mixing generally introducesunwanted air bubbles into the polymer 30.

The method next includes the step of applying a force to the liquidpolymer 30 to remove the air bubbles therefrom. Specifically, thisentails centrifuging the polymer 30. The centrifuging is performed priorto introducing the polymer 30 to the nut cavity 14.

When threads 32 are formed in large sized nuts 12, the amount of polymer30 displaced by the screw 42 may force the upper and lower supportmembers 52, 54 from engagement in the upper and lower counterbores 18,19, respectively. To prevent this problem, retaining means 70 may beprovided to retain the support members 52, 54 in engagement with the nut12. To retain the upper and lower support members 52, 54 to the nut 12,first the tie rods 80 are threaded into the threaded bores 86 on thelower support member 54. Next, the nut 12 may be lowered onto the lowersupport member 54 so that the lower cylindrical portion 56 engagestelescopingly within the lower counter bore 19 of the nut 12. Next, theupper support member 52 is lowered over the tie rods 80 so that the tierods 80 pass through the holes 84. The upper support member engagesfully with the nut 12 so that the upper cylindrical portion 68 engagestelescopingly within the upper counter bore 18. Finally, the tie nuts 90are disposed on the tie rods 80 and threaded down along the threads 82until the nuts 90 tightly abut against the upper support member 52.Retaining the support members 52, 54 to the nut is not necessary whenusing generally smaller nuts.

Once the air bubbles are removed, the polymer 30 is introduced into thenut cavity 14. Preferably, the polymer 30 is injected into the cavity14.

Since the polymer 30 shrinks during curing, steps must be taken tocompensate for the shrinkage or to reform the threads 32 aftershrinkage. If no steps are taken to minimize the effects of the polymershrinkage, the lead screw will lack proper clearance to move through thethreads 32 formed in the nut 12. Since reforming the threads 32 isdifficult and time consuming, compensating for the polymer shrinkage ispreferred. In compensating for the polymer shrinkage, it is desirable touse a casting screw 42 with larger thread dimensions than the lead screwso that when the polymer 30 shrinks, the polymer threads 32 will stillprovide enough clearance for the lead screw to pass through. The novelway of producing such a casting screw 42 is to use a length of theactual lead screw which will be used with the nut whose threads arebeing cast, and to coat this casting screw 42 with a metal coating. Thepreferred metal coating is nickel plating since the nickel coats evenlyover the threads of the casting screw 42. The nickel can be coated asthickly on the casting screw 42 as desired. The coating is usually under5/1000 of an inch, since this is the maximum coating achievable withcurrent methods. The thickness of the nickel coating depends on theamount of shrinking expected from the polymer 30.

After the polymer 30 is introduced into the nut cavity 14, the castingscrew 42 is next coated with release agent and lowered down through thescrew locating bore 62 and into the polymer. As the casting screw 42 islowered slowly down into the polymer 30, it is rotated in the threadeddirection to avoid the formation or inclusion of air bubbles in thepolymer 30. The "threaded direction" is the direction the casting screw42 would be rotated to screw the screw into a solid object.

Once the casting screw 42 is inserted in the polymer 30 the castingscrew should be aligned centrally within the nut cavity 14 in order toleave an annular space thereabout containing the polymer material 30.This involves ensuring that 1) the screw 42 passes through the upperlocating bore 62, 2) the screw passes through the lower locating bore58, and 3) the convex tip 46 of the screw 42 fits correctly into theconcave portion 57.

Next, the polymer 30 is allowed to cure in the cavity 14 of the nut 12around the threads 44 of the casting screw 42.

Once the polymer 30 has cured, the casting screw 42 may be removed fromthe nut 12. Pliers or some other suitable tool may be applied toopposing flats 48 on the head of the casting screw 42 to rotate thescrew out of the polymer insert 30. This step and the preceding step maybe interchanged.

Finally, the upper and lower support members 52,54 may be removed fromthe nut 12. The upper and lower screw support members 52,54 can bere-used in other nut casting operations.

After some finishing work on the nut 12, including machining andcleaning, the nut should be ready for use with the lead screw. Theproduct of this process is a nut 12 having polymer threads 32 which aresecurely retained within the nut and which have almost no surfaceimperfections.

What is claimed is:
 1. A method of forming threads on an interior cavityextending between top and bottom ends of a nut for engaging amulti-start high helix lead screw of a nut drive and lead screwassembly, the method comprising the steps of: closing the cavity at thebottom end of the nut to render the bottom fluid-tight; thereafterfilling the cavity of the nut with a viscous polymer material;thereafter inserting a casting screw having threads thereabout into andthrough the cavity to displace the polymer material to a high level inthe cavity; curing the polymer material and forming the high helixmultiple threads of the nut complementary to the threads of the castingscrew for providing polymer nut threads.
 2. A method as set forth inclaim 1 including the step of applying a force to the liquid polymer toremove any air bubbles therefrom.
 3. A method as set forth in claim 2including the step of centrifuging the liquid polymer to remove the airbubbles therefrom.
 4. A method as set forth in claim 1 including thestep of separating a length of lead screw from a production run of leadscrews to be used with nuts in the lead screw assemblies and using theseparated length as the casting screw.
 5. A method as set forth in claim4 including the step of coating the casting screw with a coating.
 6. Amethod as set forth in claim 5 including the step of coating the castingscrew with nickel plating.
 7. A method as set forth in claim 1 includingthe step of using as the casting screw a screw having a thread dimensiongreater than the thread dimension of the lead screw.
 8. A method as setforth in claim 1 including the step of lowering he casting screw downthrough the polymer.
 9. A method as set forth in claim 8 including thestep of rotating the casting screw in the threaded direction as thescrew is lowered slowly down through the polymer, to avoid the inclusionof air bubbles in the polymer.
 10. A method as set forth in claim 1including the step of using a nut having an upper counterbore in thecavity at the top of the nut.
 11. A method as set forth in claim 10including the step of using a nut having a lower counterbore in thecavity at the bottom of the nut.
 12. A method as set forth in claim 11including the step of coating a lower screw support member with releaseagent.
 13. A method as set forth in claim 12 including the step offitting the lower screw support member into the lower counterbore at thebottom of the cavity.
 14. A method as set forth in claim 10 includingthe step of coating an upper screw support member with release agent.15. A method as set forth in claim 11 including the step of fitting theupper screw support member into the upper counter bore at the top of thecavity.
 16. A method as set forth in claim 1 including the step ofcoating the casting screw with release agent prior to introducing thescrew into the polymer.
 17. A method as set forth in claim 16 includingthe step of retaining upper and lower screw support members inengagement with the nut whereby upper and lower cylindrical portions ofthe upper and lower screw support members respectively are retained intelescoping engagement with upper and lower counterbores respectively.18. A method as set forth in claim 16 by including the step of aligningthe casting screw centrally within the cavity with an annular spacethereabout containing the polymer.
 19. A method as set forth in claim 1including the step of allowing the polymer to cure in he cavity of thenut and around the threads of the casting screw.
 20. A method as setforth in claim 19 including the step of removing the casting screw fromthe nut after curing the polymer.
 21. A method as set forth in claim 20including the step of removing the upper and lower support members fromthe nut.